I want to to create an portrait style avatar for use on my profile on stackexchange, facebook, and other social sites. Ideally the image should be a quality head shot, recognizable over a range of sizes, and still look good when scaled down to tiny thumbnails. Hopefully the image would stand out in an array of other avatars.

I know this is a lot to ask of a couple of hundred pixels.

Are there any tips on preparing such a shot? Is there any particular lighting that works best? Are monochromes better for this task? what about cropping? Head and shoulders or face only?
Edit: the avatar should be recognizable as me.

Do you want it to look like you or do you want it to be distinguishable as your avatar?

zoom and crop don't really matter, so long as your head isn't tiny in the photo you should be okay. I've seen good profile pictures that have nothing but the eyes nose and mouth, and others that have the whole head and a background. if you do have a background make sure you can distinguish where your head end and the background begins, or not... to be honest, and I know this is a bit of a non answer, do something that looks good and experiment.

Read up on portrait photography and anything you learn there will probably apply to your Avatar photo.

As has been mentioned, treat it like a portrait shot. If you want a basic face, then you will probably want a focal length above 70mm and a background at least 30cm away from the subject. And you want to basically fill the field with the subject. This is basic stuff, really, and a good starting point for taking a portrait of any type.

Now that I've said that, getting creative means knowing what breaking any of those guidelines means. A focal length of 20mm will produce a distinct effect, for example, which might be artistically desired. Putting the subject off-centre, so you get half their face in half the image would be a different artistic decision, but one that makes the background more important. And colour-casts (including monochrome) and other effects take artistic in yet another direction but do have the advantage of helping the image stand out amongst a lot of others.

Finally, it is possible for a completely non-portrait picture to work effectively as an avater image. Mine is one such. :-)